The present invention relates to a method for generating superheated steam by means of the heat which is generated in the boiler of an incineration plant. The invention further relates to an apparatus for generating superheated steam by means of the heat which is generated in the boiler of an incineration plant and an incineration plant comprising such an apparatus.
Incineration plants are known to the person skilled in the art and can be used, for instance, for the combustion of waste. Such incineration plants generally have a boiler which comprises a combustion chamber for burning the material which is intended to be burnt and boiler passes which are adjacent thereto in the flow direction of the flue gas.
In the boiler passes, there are arranged at least partially heat exchangers which are configured, on the one hand, to recover in a profitable manner the highest possible proportion of the thermal energy which is released during the combustion and, on the other hand, to cool the components of the incineration plant which have been acted on by the hot flue gas and the flue gas itself.
Generally, in these heat exchangers, water is used in the liquid or gaseous state as a heat exchanger fluid. The steam which is produced when water is used can, for instance, be used to produce electrical power.
In this case, the production of steam in order to produce electrical power is carried out in principle in three stages. In a first stage, the water is heated in the liquid state up to a maximum corresponding to the evaporation temperature. In a second stage, the water is evaporated, thus brought from the liquid into the gaseous state, without the temperature increasing in this case. In a third stage, the steam is then superheated, that is to say, it is further heated to a temperature which is above the evaporation temperature, before it is ultimately supplied to a steam turbine for power production.
In accordance with these three stages, pre-heating, evaporation and superheating, incineration plants have different heat exchangers. Whilst the evaporators which are configured for the evaporation are often constructed in the form of diaphragm walls, that is to say, as pipes which are securely connected to each other by means of webs, the pre-heaters and superheaters which are configured for the pre-heating or superheating are generally in the form of pipe bundles or tubular coils in the case of horizontal heat exchangers and in the form of pipe rows (or “harps”) which are connected via collectors in the case of suspended heat exchangers.
Whilst the energy from flue gas in a high temperature range above 500° C. is transmitted to the heat exchanger fluid substantially by means of radiation, it is carried out at lower temperatures substantially by means of convection. Accordingly, the boiler passes are sub-divided into a radiation portion (or “radiation passes”) and a convection portion (or “convection passes”).
However, particularly with superheaters which are generally arranged in the convection portion, it should be ensured that they are not subjected to excessively high wall or material temperatures. It is thus possible, in particular with a flue gas temperature which is substantially above 600° C., that corrosion problems may occur. These result from the fact that at such high flue gas temperatures still-gaseous corrosion-inducing flue gas components condense on the cooler pipe walls or the coatings on the pipe walls and are available at that location for corrosion processes. As a result of this limitation with regard to the application temperature, superheaters are generally operated, on the one hand, at flue gas temperatures less than 650° C. and, on the other hand, superheating temperatures less than approximately 430° C. are desired.
Nonetheless, there are proposed concepts which are intended to prevent corrosion problems even at higher superheating temperatures:
The general problem of minimizing the corrosion on heat exchanger pipes in a steam boiler installation is combated, for instance, according to DE-A-102010032612 in that the heat exchanger pipes are surrounded by ceramic material.
There is further described in JP 2000297613 a method in which a secondary medium is heated by means of a heat exchanger which is located in the flue gas flow and the secondary medium is used as a heat source for superheating water steam. Since the secondary medium has a very much lower pressure than the steam which is intended to be produced, pipes of conventional boiler steels or ceramic material can be used in the heat exchanger. In order to obtain adequate superheating, the heat exchanger surface-areas which are required in accordance with the technology of JP 2000297613 are, however, very large and consequently cost-intensive.
In addition in EP-A-2423584 there is proposed a steam producer which contains a wall superheater which comprises a plurality of pipelines through which superheated steam flows and which are protected from the flue gas which is produced during the combustion process by means of plate-like elements of a corrosion-resistant material.
Furthermore, EP-A-2011972 relates to the problem of using the thermal value of the fuels at a higher exergetic level and consequently enabling an increase of the degree of electrical efficiency without the boiler being endangered by corrosion. To this end, it is proposed to incorporate in the walls of a combustion chamber which are built up with a fire-resistant material channel-like cavities in order to guide a gaseous heat-carrying medium which is directed afterwards to the superheater and which is used at that location to superheat the steam.
Reference is further made to EP-A-0981015 which relates to the problem of providing a steam producer for superheated steam for an incineration plant with corrosive flue gases in which a high superheating temperature can be achieved without corrosion at the final superheater. To this end, it is proposed in EP-A-0981015 that plates be arranged on a wall of the radiation portion, wherein there is provided between the plates and the wall a space in which at least a portion of a superheater is arranged as a wall superheater and which contains a non-corrosive gaseous atmosphere. In this instance, the gaseous atmosphere flows through the space in the lowest possible quantity.
A method for protecting heat exchanger pipes which extend freely in the furnace and which are suspended on the ceiling of the furnace is further disclosed in DE-A-102013000424; according to this method, a gas is also supplied between the heat exchanger pipe and the ceramic element which surrounds the heat exchanger pipe.
The disadvantage of the technology described in EP-A-0981015 is that the energy balance which is produced is poor:
Although according to EP-A-0981015 the quantity of air flowing through the corrosion-free space is intended to be kept as low as possible, in practice a portion of the heated air always flows via gaps, cracks or pores into the combustion chamber. Taking into account the large surface-area required and the relatively high excess pressure which has to be maintained in the corrosion-free space, the quantity of heated air which can be discharged into the combustion chamber is consequently relatively high. The thermal energy contained in the heated air is consequently guided into the combustion chamber in the unused state, whereby the flue gas volume flow and therefore the flue gas losses of the boiler are increased, which is unfavorable from the point of view of energy.
Furthermore, the technology in accordance with EP-A-0981015 has the disadvantage that the final superheating or the final superheated quantity of steam cannot be controlled in any way.